Pulse transformers are those designed to handle nearly rectangular wave forms. A common and important application is the coupling of a load resistance to a source of pulsed power. Radar transmitters, for instance, usually employ an output power tube such as a magnetron, which must be driven at a relatively high voltage and high impedance level. The pulse modulator, however, generates pulses at relatively low levels of voltage and impendance. The output impedance of the modulator is limited to a region defined by the maximum voltage and peak current ratings of the pulse switch which is used. It is customary to couple the modulator to the load with the necessary impedance transformation by using a pulse transformer having the appropriate step-up ratio.
If Z.sub.o is the optimum output impedance of the modulator and R.sub.LOAD is the static load resistance of the power tube (usually a magnetron) at its operating point, then the required step-up ratio of the pulse transformer, r, is given by ##EQU1## For a given pulse duration, .delta., the total inductance, L, in the modulator pulse-forming circuit is ##EQU2## The quantity, L, includes the distributed inductance of the leads and the total leakage inductance of the pulse transformer referred to the primary.
From Eq. 2 it can be seen that L is proportional to the quantity .delta./r.sup.2. Where .delta./r.sup.2 is small, as for short pulses and/or large step-up ratios, then L becomes small also. It then becomes a problem to design a pulse transformer whose distributed inductance plus leakage inductance is smaller than the required value of L.
It is known to make inductive windings for transformers from metallic foils, sheet or strip as well as wire. Since turn to turn voltages are relatively low, insulation is provided by a thin strip of material, such as kraft paper wound between the conductor turns at the time the winding is constructed. A coating of insulating enamel has also been used.
To minimize eddy current losses in sheet wound transformers, it is common to subdivide the conducting sheet into two or more elements connected in parallel at the beginning and end of the windings. In sheet type windings this is done by winding a number of sheet conductors simultaneously in a superposed relation. The parallel loops, however, introduce losses due to circulating currents. These losses are minimized by transposing the loops at various points on the winding. In the case of sheet conductors, this presents a difficult manufacturing problem, requiring intricate notching and arrangement of the sheets. Another drawback of this technique is increased heating due to the smaller conducting area.
This problem does not exist in the pulse transformers for which this invention is applicable since the average current requirements are relatively low. Typically, the winding may be wound of copper sheet 1-2 mils thick. Eddy current losses in the sheet are proportional to the square of its thickness and are extremely small for 1 mil material. Therefore, it is not necessary nor desirable to use a plurality of parallel sheets insulated from each other and connected together at the ends of the winding.
A sheet wound transformer also develops eddy currents and distributed inductance in nearby conductors such as the lead-in conductors. These currents are sometimes reduced by pairing lead-in conductors in opposite directions of conductance. It has generally been thought necessary to employ the subdivided winding arrangement described above in order to get the lead-in conductors to share the current equally.
When a subdivided winding with multiple lead-in conductors is employed, the method of connecting lead-in conductors to the conductor windings is more complex than the standard method whereby a single lead-in conductor is attached at opposite ends of both (or all) of the parallel windings. A standard lead-in conductor is a thin metal strip such as copper attached as by soldering to the parallel conductors so as to extend along their width.
Stripline transmission material consists of two conductors separated along their length by a dielectric material which allows the conductors to interact. It has been used to form the parallel windings of a transformer, as in U.S. Pat. No. 3,611,233. It has not, however, to the inventors's knowledge, been previously used for coupling lead-in connections nor has it been effectively employed to couple lead-ins to a single foil winding to reduce distributed inductance.